1. Field of the Invention
The present invention relates to improvements in NEC 9801/IBM-AT double compatibility computers using a modified Industry Standard Architecture (ISA) compatible bus. The invention involves a method and apparatus for disabling certain individual slots on an ISA compatible bus when the double compatible computer operates in the NEC 9801 compatible mode.
2. Description of the Related Art
The ISA bus is one of the most common standards for desk top computers in the United States. Such desk top computers are often referred to as IBM-AT compatible computers. The ISA bus provides communications between an Intel 80.times.86 microprocessor and a plurality of add-on cards that provide a plurality of functions (e.g., hard disk controllers, floppy disk controllers, modems, video display controllers, etc.). A number of manufacturers design and sell cards that are compatible with the ISA bus and that can be inserted into so-called expansion slots on the ISA bus.
The Intel 80.times.86 microprocessors are also used in computers in Japan. For example, the NEC 9801 computer utilizes such microprocessors. Many compatibilities exist between the IBM AT compatible computer standard and the NEC 9801 computer. These compatibilities make it possible to develop a double-compatible computer. However, numerous incompatibilities exist particularly involving Input/Output (I/O), I/O busses and peripheral devices. Some hardware incompatibilities can be solved by using hardware which is mode selectable. However, in many cases, this requires redundant hardware, each set operating in one mode or the other. This is impractical and undesirable. Therefore, complete hardware compatibility is not practical, and thus, one basic hardware bus should be chosen.
If the ISA (AT) bus is chosen for the dual-compatible computer, then only ISA type add-on cards are mechanically and electronically compatible. NEC 9801 add-on cards will not function with the ISA bus. Many ISA compatible add-on cards function properly in the IBM-AT compatible mode and do not interfere with operation of the computer in the 9801 compatible mode (hereinafter referred to as non-interfering cards). However, some ISA cards (hereinafter referred to as interfering cards) which function properly in the IBM compatible mode will interfere with computer operations when the computer operates in the 9801 mode because conflicts exist between devices on the cards and the standard peripheral devices required by the NEC 9801 compatible system. Thus, bus collisions will occur when an interfering ISA card responds to an address which conflicts with the standard peripheral actually requested in the NEC 9801 emulation mode. This can result in a computer misfunction.
These incompatibilities could be solved by designing intelligent add-on cards which do not respond if address conflicts exist and collisions will occur on the bus in the NEC 9801 compatible mode. However, this would require redesigning all ISA (AT) add-on cards to include circuitry to disable the card when the computer operates in the NEC 9801 and an address conflict exists. Many add-on cards are commercially available and are widely used in the United States. It would be impractical and uneconomical to redesign them all just for use in the dual compatible computer; moreover, users often have substantial investment in hardware and would not wish to replace all existing cards with new intelligent cards in order to use the dual compatible computer.
Another solution is for the user to actually remove interfering cards from the bus when using the computer in the NEC 9801 compatible mode. This is very inconvenient because it would require the user to open the computer and remove or insert cards each time the mode of the computer changes. This also presents a risk of damage to the computer from repeated removal and installation of add-on cards.
Tomcat Company, a Japanese corporation, attempted to solve this problem by simply placing jumpers for each slot within the computer which, when removed, would disable a card plugged into the corresponding ISA bus slot. Although this may be preferable over removing the entire card, the user must still open the computer and physically remove the jumper each time the computer mode changes. This may reduce the risk of damage to the computer, but still involves significant inconvenience. Therefore, this attempt to solve the problem was not feasible.
A reasonable solution to this problem would utilize control to automatically disable a slot containing an interfering card whenever the computer operates in the NEC 9801 compatible mode and enable the slot whenever the computer operates in the AT compatible mode. This would prevent any misfunctions and allow the computer to use standard, unaltered, ISA compatible cards without interferences when the computer operates in the 9801 compatible mode.
Implementing a solution to allow use of existing ISA compatible cards without interferences in the computer would dramatically improve the feasibility of IBM AT/NEC 9801 dual-compatible computers.